The present invention relates to a method and apparatus for achieving multiplexing/demultiplexing of wavelengths, while concentrating wavelengths within a channel into a discrete location, particularly by cascading two like dispersion steps, with a local inversion of wavelengths about a channel center wavelength between them.
In wavelength division multiplexed optical communication systems, many different optical wavelength carriers provide independent communication channels in a single optical fiber. Future computation and communication systems place ever-increasing demands upon communication link bandwidth. It is generally known that optical fibers offer much higher bandwidth than conventional coaxial communications; furthermore a single optical channel in a fiber waveguide uses a microscopically small fraction of the available bandwidth of the fiber (typically a few GHz out of several tens of THz). By transmitting several channels at different optical wavelengths into an fiber (i.e., wavelength division multiplexing, or WDM), this bandwidth may be more efficiently utilized.
There have been many attempts to develop a compact, high-resolution waveguide demultiplexor or spectrometer for application in areas such as spectroscopy, optical networks and optical links and more particularly optical communication systems. Such a demultiplexor can be extremely critical in wavelength division multiplexing (WDM) links. In these links or networks, each channel is assigned a distinct and unique wavelength for data transmission. Thus, the optical fiber that connects channels in a WDM network carries many discrete wavelength channels and a particular wavelength is selected before the data is received. The data reception can be achieved by combining a wavelength demultiplexor, photodetectors and electronic selection circuitries. In WDM links, many wavelengths are multiplexed and transmitted through a single optical fiber to increase the capacity of the fiber. The receiver must demultiplex the many wavelengths and select the proper channel for reception. In these applications, the requirements on the wavelength demultiplexor are typically: an optical bandwidth greater than 30 nm, a wavelength resolution of a few angstroms, polarization insensitivity, compactness, low loss, low crosstalk, and a low manufacturing cost.
At present, there are many known methods of selecting particular wavelengths, however, none are ideal for the applications outlined above.
Techniques for multiplexing and demultiplexing between a single optical fiber comprising the multiplexed channel and plural optical fibers comprising the plural demultiplexed channels are described in various U.S. patents. For example, multiplexing/demultiplexing with birefringent elements is disclosed in U.S. Pat. Nos. 4,744,075 and 4,745,991. Multiplexing/demultiplexing using optical bandpass filters (such as a resonant cavity) is disclosed in U.S. Pat. Nos. 4,707,064 and 5,111,519. Multiplexing/demultiplexing with interference filters is disclosed in U.S. Pat. Nos. 4,474,424 and 4,630,255 and 4,735,478. Multiplexing/demultiplexing using a prism is disclosed in U.S. Pat. No. 4,335,933. U.S. Pat. No. 4,740,951 teaches a complex sequence of cascaded gratings to demultiplex plural optical signals. U.S. Pat. Nos. 4,756,587 and 4,989,937 and 4,690,489 disclose optical coupling between adjacent waveguides to achieve a demultiplexing function. A similar technique is disclosed in U.S. Pat. No. 4,900,118. Although some of these techniques are better than others, there is a need for a system that provides demultiplexed channels having spectral amplitudes that are as flat as possible.
Methods have been employed wherein an optical element having a periodically varying spectral response is used to flatten the spectral amplitudes within each of a group of multiplexed channels having different predetermined central wavelengths with uneven spectral amplitudes peaked around the central wavelengths. However such techniques and attempts a flattening reduce the overall power by xe2x80x9cchoppingxe2x80x9d the signal to obtain flatter spectral amplitudes or xe2x80x9cflat topsxe2x80x9d.
Dispersion elements commonly used in the prior art for demultiplexing channel separation cause continuous dispersion of individual wavelengths. However, each channel band for demultiplexing includes a plurality wavelengths dispersed continuously with the wavelengths of the other channel bands. As a result, the position of all wavelengths within a channel band to be picked up are not the same, and full amplitude pick up of all wavelengths by a waveguide or optical fiber is not possible. Further, the continuous dispersion causes difficulty of separation of individual channels causing crosstalk.
It is desired to provide demultiplexing by dispersing a complete channel band to a single position for the pick up of a signal without excess loss. It is also desired to separate individual channel bands from each other to reduce cross talk.
In accordance with the invention, there is provided a method of demultiplexing an optical beam into a plurality of channels, comprising a plurality of wavelengths of light, comprising the steps of:
providing the optical beam to a first dispersive means, to demultiplex the beam into spatially dispersed wavelengths of light;
dividing the spatially dispersed wavelengths of light into channels;
performing a inversion of the spatially dispersed wavelengths of light, centered about a center wavelength within each channel, to spatially invert their positions; and
providing the inverted wavelengths of light to one of the first dispersive means and another dispersive means providing substantially the same dispersion as the first dispersive means for the plurality of channels, to provide the demultiplexed channels.
In accordance with a further embodiment of the invention an optical device is provided for demultiplexing an optical beam into a plurality of channels, comprising a plurality of wavelengths of light, comprising:
a first dispersive grating for receiving the beam to demultiplex the beam into spatially dispersed wavelengths of light;
means for dividing the spatially dispersed wavelengths of light into channels and for inverting the spatially dispersed wavelengths of light centered about a center wavelength within each channel; and
dispersive means for receiving the inverted wavelengths of light to provide the demultiplexed channels.
In a still further preferred embodiment of the invention an optical device is provided for demultiplexing an optical beam into a plurality of channels, comprising a plurality of wavelengths of light, comprising:
a first phased array grating for receiving the beam to demultiplex the beam into spatially dispersed wavelengths of light on a first output focal plane;
means for dividing the spatially dispersed wavelengths on the first output focal plane into channels and for inverting spatially dispersed wavelengths centered about a center wavelength within each channel;
a second phased array grating having an input focal plane for receiving the inverted wavelengths of light, for demultiplexing the channels.
Advantageously, demultiplexing in accordance with the present invention provides dispersion of a multiplexed signal to discrete channel locations reducing pick up loss and crosstalk.